1. Field
One or more aspects of embodiments according to the present invention relate to swept local oscillators and more particularly to a swept local oscillator system with high spectral purity output.
2. Description of Related Art
Local oscillators are used in numerous radio-frequency applications, for example for frequency translation in a dual conversion receiver. The spectral purity of a local oscillator may have significant consequences for system performance. For example, if the radio frequency (RF) input of a receiver receives a combination of a desired narrow-band signal and undesired broadband noise or interference, the undesired mixing products of the received noise or interference with spectral impurities in the local oscillator may fall in the same intermediate frequency (IF) band as the desired mixing products of the desired received signal and local oscillator signal. Local oscillator spectral impurities may be the result of phase noise or may be spurious spectral components caused by other mechanisms. If a local oscillator employs direct digital synthesis (DDS), for example, spurious spectral components may form as a result of various nonlinear mechanisms, for example in the digital to analog converter (DAC) of the DDS oscillator, where these mechanisms may produce intermodulation products of the clock frequency and its harmonics with the oscillator frequency and its harmonics.
In a swept frequency DDS oscillator the spurious spectral content may be particularly troublesome because its spectrum, relative to the frequency, or instantaneous frequency, of the oscillator will change as the frequency is swept, and the frequencies of the intermodulation products change depending on their order. DDS oscillators have important advantages, however in swept local oscillator applications, including frequency agility. Thus, there is a need for a system using a DDS oscillator to form a high spectral purity swept local oscillator.